1. Field of the Invention
The present invention relates to a semiconductor device and more particularly to a static type semiconductor memory device.
2. Description of the Related Art
Heretofore, different types of semiconductor devices, which can store binary information or data in terms of electric charges, have been developed and are at present used in various memories such as static memories, dynamic memories, read only memories (ROMs), or the like. The reason for this is that a semiconductor memory having high integration density and large storage capacity can be easily obtained because each memory cell can be formed with a very small area. Among these memories, the static type semiconductor memory has been widely used as a random access memory (RAM), because it can retain stored data without periodical refresh operation.
The static random access memory (SRAM) can be implemented by a large number of flip-flop circuit each of which stores one bit of information. Recently, a CMOS flip-flop circuit which is composed of a pair of N-channel MOS field effect transistors and a pair of P-channel MOS field effect transistors is widely utilized as a memory cell because power consumption of the CMOS flip-flop circuit is very small. However, the CMOS flip-flop circuit generally necessitates a relatively large area on a semiconductor substrate, and therefore it is difficult to fabricate a large capacity of SRAM by employing the CMOS flip-flop.
In order to solve the above problem, it has been proposed to form a pair of P-channel MOS transistors of the CMOS flip-flop circuit as thin film transistors (TFTs) or by silicon-on-insulator (SOI) structure. According to the TFT or SOI technique, the N channel MOS transistors are formed at a surface of a semiconductor substrate and the P-channel MOS transistors are fabricated by a polycrystalline silicon layer or a monocrystalline silicon layer which is formed on the surface of the semiconductor substrate via an insulating layer. According to this technique, the P-channel transistors can overlap a part of the N-channel MOS transistors, and therefore the integration scale of SRAM is enlarged. However, crystalline characteristic of the polycrystalline silicon film or the monocrystalline silicon film formed on the semiconductor substrate via an insulating layer is not good and a PN junction formed in the above silicon film is very leaky. Therefore, power consumption of the SRAM employing the above SOI type transistors is relatively large.